The present invention relates to a twin asymmetric scan slice thickness setting method and apparatus and a radiation tomography method and apparatus, and more particularly to a method of setting slice thicknesses for a twin asymmetric scan in which a scan is performed with a first slice thickness in a first detector row of a multi detector and with a second slice thickness different than the first slice thickness in a second detector row, and an apparatus which can suitably implement the method.
In a conventional X-ray CT apparatus comprising a twin detector having first and second detector rows, a slice thickness setting method is known for a twin symmetric scan in which a scan is performed with the same slice thickness in both the first and second detector rows.
The slice thickness setting method for the twin symmetric scan involves moving a position of an X-ray beam and performing a scan while keeping the width of the X-ray beam fixed at twice the slice thickness to move the position of the X-ray beam to a position at which count values for the first and second detector rows match.
However, a slice thickness setting method for a twin asymmetric scan in which a scan is performed with a first slice thickness in a first detector row and with a second slice thickness different than the first slice thickness in a second detector row is not known in the prior art.
It is an object of the present invention to provide a twin asymmetric scan slice thickness setting method and apparatus for setting slice thicknesses for the aforementioned twin asymmetric scan, and a radiation tomography method and apparatus which can suitably implement the method.
In accordance with a first aspect of the present invention, there is provided a twin asymmetric scan slice thickness setting method for performing a scan with a first slice thickness in a first detector row of a multi detector and with a second slice thickness different than the first slice thickness in a second detector row, comprising the steps of moving a position of a penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at twice the first slice thickness to move the position of the penetrating radiation beam to a position at which count values for the first and second detector rows match, storing the count value for the detector row at this time, and then moving the position of the penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at the sum of the first and second slice thicknesses to move the position of the penetrating radiation beam to a position at which the count value for the first detector row matches the stored count value.
The sensitivity profile of each of the first and second detector rows in the slice thickness direction has an inverse U-like shape having the crest in the center, gradually falling toward the ends (see FIG. 1(b)). Hence, a relation (first slice thickness:second slice thickness)=(count value for the first detector row:count value for the second detector row) does not stand. That is, the ratio between the count values cannot determine the position of the penetrating radiation beam when the first slice thickness the second slice thickness.
However, since the sensitivity profiles of the first and second detector rows in the slice thickness direction can be considered to be symmetric as viewed from the boundary between the first and second detector rows, a relation (count value for the first detector row count value for the second detector row) stands when the first slice thickness=the second slice thickness. That is, the position of the penetrating radiation beam can be determined by the ratio between the count values (1:1) when the first slice thickness=the second slice thickness.
The present invention utilizes the above-described principle.
More specifically, according to the twin asymmetric scan slice thickness setting method of the first aspect, first, a position of the penetrating radiation beam is moved and a scan is performed while keeping the width of the penetrating radiation beam fixed at twice the first slice thickness to move the position of the penetrating radiation beam to a position at which count values for the first and second detector rows match (i.e., to a position at which the ratio of the count values is 1:1). At this time, the slice thickness in the first detector row and the slice thickness in the second detector row are equal to the first slice thickness. Then, the count value f or the detector rows at this time is stored. Next, the position of the penetrating radiation beam is moved and a scan is performed while keeping the width of the penetrating radiation beam fixed at the sum of the first and second slice thicknesses to move the position of the penetrating radiation beam to a position at which the count value for the first detector row matches the stored count value. At this time, the slice thickness in the first detector row is equal to the first slice thickness. On the other hand, since the slice thickness in the second detector row has a value subtracting the first slice thickness from the sum of the first and second slice thicknesses, the slice thickness in the second detector row is equal to the second slice thickness. The slice thicknesses can thus be set for a twin asymmetric scan.
In accordance with a second aspect of the present invention, there is provided a twin asymmetric scan slice thickness setting method for performing a scan with a first slice thickness in a first detector row of a multi detector and with a second slice thickness different than the first slice thickness in a second detector row, comprising the steps of: moving a position of a penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at twice the first slice thickness to move the position of the penetrating radiation beam to a position at which count values for the first and second detector rows match, storing the count value for the detector row at this time, and then moving the position of the penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at the sum of the first and second slice thicknesses to move the position of the penetrating radiation beam to a position at which the count value for the first detector row matches the stored count value, storing a ratio a between the count values for the first and second detector rows at this time; prior to performing an actual scan of a subject, acquiring a count value for a first control detector disposed corresponding to the first detector row and a count value for a second control detector disposed corresponding to the second detector row, and moving a position of the penetrating radiation beam to a position at which a ratio xcex2 between these count values matches the stored ratio xcex1.
According to the twin asymmetric scan slice thickness setting method of the second aspect, the slice thicknesses are set for a twin asymmetric scan based on the twin asymmetric scan slice thickness setting method as described regarding the first aspect. Then, a ratio a between the count values for the first and second detector rows at this time is stored. Thereafter, and prior to performing an actual scan of a subject, a count value for a first control detector disposed corresponding to the first detector row and a count value for a second control detector disposed corresponding to the second detector row are acquired, and the position of the penetrating radiation beam is moved to a position at which a ratio xcex2 between these count values matches the stored ratio xcex1. Thus, if the reference ratio xcex1 is obtained by performing the twin asymmetric scan slice thickness setting method of the first aspect only once, the ratio can be used as many times as desired in the subsequent actual scans, thereby simplifying the operations for setting the slice thicknesses for an asymmetric scan as a whole.
In accordance with a third aspect of the present invention, there is provided a radiation tomography apparatus comprising a multi detector having a first detector row having a first slice thickness and a second detector row having a second slice thickness, comprising means for acquiring a count value corresponding to the first slice thickness, for moving a position of a penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at twice the first slice thickness to move the position of the penetrating radiation beam to a position at which count values for the first and second detector rows match, storing the count value for the detector row at this time; and means for setting a position of the penetrating radiation beam, for moving the position of the penetrating radiation beam and performing a scan while keeping the width of the penetrating radiation beam fixed at the sum of the first and second slice thicknesses to move the position of the penetrating radiation beam to a position at which the count value for the first detector row matches the stored count value.
The radiation tomography apparatus of the third aspect can suitably implement the twin asymmetric scan slice thickness setting method as described regarding the first aspect.
In accordance with a fourth aspect of the present invention, there is provided a radiation tomography apparatus comprising a multi detector having first and second detector rows, comprising means for storing a reference ratio, for storing a ratio xcex1 between a count value for the first detector row having a first slice thickness and a count value for the second detector row having a second slice thickness different than the first slice thickness; and means for setting a position of a penetrating radiation beam, for, prior to performing an actual scan of a subject, acquiring a count value for a first control detector disposed corresponding to the first detector row and a count value for a second control detector disposed corresponding to the second detector row, and moving a position of the penetrating radiation beam to a position at which a ratio xcex2 between these count values matches the stored ratio xcex1.
According to the radiation tomography apparatus of the fourth aspect, a ratio xcex1 between the count values for the first and second detector rows is stored when the slice thicknesses are set for a twin asymmetric scan based on the twin asymmetric scan slice thickness setting method as described regarding the first aspect, and the ratio is used in an actual scan to set the slice thicknesses for the asymmetric scan. Thus, the stored ratio xcex1 can be used as many times as desired in the subsequent actual scans, thereby simplifying the operations for setting the slice thicknesses for an asymmetric scan as a whole.
Therefore, according to the twin asymmetric scan slice thickness setting method and apparatus and a radiation tomography method and apparatus of the present invention, slice thicknesses can be set for a twin asymmetric scan in which a scan is performed with a first slice thickness in a first detector row of a multi detector and with a second slice thickness different than the first slice thickness in a second detector row.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.